1. Technical Field
The present invention relates to motors for driving devices, e.g., cooling fans. More particularly, the present invention relates to motors for driving devices, e.g., cooling fans, for use in electronic systems and for software emulation and compensation within the driven devices to achieve efficient operation.
2. Discussion of the Related Art
In electronic systems, such as computer systems, cooling fans play an important role in maintaining the system's capabilities. The inability to remove excessive heat from electronic systems may lead to permanent damage of the system. Because of the complexity of existing electronic systems, cooling fans have added functionalities other than just providing cooling air, such as the ability to control the speed of a fan, the ability to monitor a tachometer pulse on a fan to determine instantaneous fan speed, and the ability to detect if a fan has failed or is slower than its preset speed.
Brushless D.C. motors, utilized today in electronic devices such as personal computers, servers, laptops, and desktop computers, include a rotating permanent magnet rotor, a stator carrying field coils, and a drive circuit for sequentially exciting the field coils with digital pulses, thereby creating electronic commutation. Electronically commutated motors eliminate or reduce the disadvantages inherent in motors with mechanical structures for a commutator. Specifically, radio frequency interference losses and electromagnetic induction (EMI) losses are reduced or eliminated. Power consumption attributed to armature-brush arcing is also eliminated.
The electronic driving circuitry incorporates active electronic components such as MOSFETs, to provide pulse width modulated signals. Drive pulse generation is synchronized with rotor position by the incorporation of monitoring or feedback circuitry, including the use of position sensors, such as Hall effect devices. In a cooling fan, a circuit board may include a position sensor, such as a hall effect sensor, a rotor, a stator, and a microcontroller.
A DC brushless motor may include a driving circuit based on an integrated circuit that is soldered to the printed circuit board. The printed circuit board is contained inside a motor casing together with the stator and the rotor. The rotor, supported by bearings, may thus be propelled inside the stator to produce mechanical rotation under control of the electronic driving circuitry. A stator is comprised of a selected number of turns of conductor coils wound around a bobbin reel.
In operation, the stator field winding acts as an electromagnetic source that produces magnetic flux as a result of the excitation current it receives. The generated magnetic flux flows in a longitudinal direction of the generally cylindrical-shaped body of the stator along the magnetic circuit inside the upper and lower magnetic pole pieces, which are located at both ends of the stator cylindrical body. The magnetic flux flows either into or out from the pole plates of the pole pieces respectively and then into or out from the rotor. Depending on the polarity of the current excitation in the stator field windings, the flux passes across an air gap in the radial direction either to or out from the corresponding magnetic poles in the ring magnet of the rotor. As the magnetic flux passes through closed loops of magnetic circuits formed in the stator, the air gap and the rotor, mechanical driving force is developed and the rotor is propelled to rotate.
Depending on the relative angular position of the rotor, the drive circuit alternatively feeds driving current of clockwise or counter-clockwise orientation into the stator field winding. As a result, the pole plates and the upper and lower magnetic pieces respectively may become alternatively energized as north and south poles respectively. With proper driving control, the rotor can thus be propelled to rotate by the stator in the desired direction and speed of rotation.
Apparatus for sensing angular position and speed of a rotary shaft have moved to utilizing less expensive components. Hall sensors and magnorestrictive sensors may be utilized to generate electrical signals when exposed to a rotating magnetic field. Hall effect sensors utilize a current-carrying semiconductor membrane to generate a low voltage perpendicular to the direction of current flow when subjected to a magnetic field normal to the surface of the membrane. The rotation of the rotor's magnetic field is detected by, for example, a Hall sensor (Hall generator circuit) which senses the rotor position and speed and provides synchronizing pulses.
If a hall sensor is placed in a neutral position, as illustrated in FIG. 1(a), the fan does not run efficiently and in some cases may not be able to reach the higher speeds required to cool the electronics or computing device enclosure. In order to combat this inefficiency, the hall sensor may be physically moved to an advanced position, as illustrated in FIG. 1(b), so that the hall sensor is activated slightly before the pole pass. The hall sensor interrupt is sent to a microcontroller, and the microcontroller switches the outputs, slightly before the magnet passes the actual pole. The switching of outputs is referred to as switching or commutation. This allows the fan to reach the higher speeds and operate in an efficient manner.
The amount of advancement of the hall sensor depends on the operating speed of the motor. Conventionally, the appropriate advancement of the hall sensor is based on the maximum motor speed. Unfortunately, this means that when the speed control of the fan is being utilized and the fan is running at a lower speed than maximum, the motor is not running as efficiently as possible.
Problems arise when the hall sensor is physically advanced as described above. A permanently advanced hall sensor causes an “oscillating effect” problem on startup and also may cause a locked rotor condition. The net result of the “oscillating effect” can prevent the fan from starting up or from rotating. The “oscillating effect” is illustrated in FIGS. 1(c) and 1(d). If the hall sensor is placed in a neutral position as shown in FIG. 1(c), the rotational torque for the motor shaft and the hall sensor is in one direction, whether momentum is present or not. If the hall sensor is placed in an advanced position, as illustrated in FIG. 1(d), a negative torque and a positive torque exist. If the fan is rotating, momentum overcomes the negative torque. If there is no momentum, such as during startup, the positive and the negative torques can pull back and forth indefinitely, causing an “oscillating effect.” The “oscillating effect” frequency is determined by the Gauss of the hall sensor, and could be interpreted by the microcontroller as the frequency of a running fan.
In addition, different designs of the cooling fan may result in hall placement changes. If a design of a cooling fan needs to be changed, the hall advancement angle or advancement amount from a neutral position may need to change to achieve max efficiency or desired speed. That results in a PCB layout change.
For example, if a fan is running at 4000 RPM, then the hall sensor or speed sensor may be adjusted to 3 degrees from neutral position, and the PCB is laid out to allow this position. If the same or similar fan needs to run at 6000 RPM, the PCB would need to be reconfigured to adjust the hall sensor or speed sensor to 5 degrees from neutral position for maximum efficiency. The design change or new circuit board layout results in an increase in monetary and time costs.
Accordingly, a need exists to allow the fan to operate at high speeds and to run in an efficient manner, while minimizing the “oscillating effect” and minimizing any additional costs associated with board layout changes.